Variable impedance generator

ABSTRACT

A photoconductive device is positioned adjacent to a light source with a radiation barrier between them. The radiation barrier has slots, and is rotated, causing light to alternately impinge upon the photoconductive device and to be blocked resulting in alternating decreases and increases in the impedance of the device. By using slots of random width and random spacing, the impedance variation introduced into a circuit simulates noise.

United States Patent Schillin Mar. 7 1972 [54] VARIABLE IMPEDANCE GENERATOR FOREIGN PATENTS OR APPLICATIONS Inventor! Arthurfl s chill ing sggtllpaytgn Elg. 1,057,030 10/1953 France ..250/233 [73] Assignee: General Electric Company a Primary Examiner M H. Grieb [22] Filed: May 22,1969 Attorney-Raymond H. Quist, Allen B. Amgott, Henry W. I Kaufmann, Joseph B. Forman, Frank L. Neuhauser and Oscar [21] Appl. No.. 826,983 B. wadde" [52] US. CL ..35/13, 250/233, 340/384 ABSTRACT A photoconductive device is positioned adjacent to a light 25b/233f346/384 384 source with a radiation barrier between them. The radiation barrier has slots, and is rotated, causing light to alternately [56] References cued impinge-upon the photoconductive device and to be blocked resulting in alternating decreases and increases in the im- UNITED STATES PATENTS pedance of the device. By using slots of random width and ran- 2 977 584 3/1961 S 1 340/384 dom spacing, the impedance variation introduced into a C11- lege cuit simulates noise 3,052,881 9/1962 Windels .....340/384 3,394,264 7/ 1968 Busey ..250/233 2Claims, 2Drawing Figures ANA 06 30A SOURCE Q IZA IOA I4B 50B ANALOG SOURCE w Q y IOB ANALOG u SOURCE 12C 10c Patented March 1, 1972 3,647,925

2 ShGBtS-ShGOt 2 IN VENTOR.

Arthur G. Schilling ATTORNEY,

1 VARIABLE IMPEDANCE GENERATOR BACKGROUND OF THE INVENTION This invenn'on relates generally to a variable impedance generator, and more particularly, to a system for simulating the noise normally found in electrical circuits.

As systems such as aircraft, power generating plants, and the like, become larger and more complex, it I becomes uneconomical to train operators through experience on the actual system. Simulators which appear to the operator to be the actual operating system are being, employed for these training purposes not only because of the economies in so doing, but also because they permit training in emergency procedures which might be hazardous with a real system.

In producing a nuclear power plant simulator it was noticed that the analog indicating devices, i.e., meters, did not accurately simulate those in the real power plant because the indicator needles failed to tremor or kick in the manner of the In accordance with the invention, the noise generator illustrated in FIG. 2 was employed. Slotted disc 16 constituting a radiation barrier having radiation transmitting portions is mounted in enclosure 18 so that radiation from light source 20 will impinge upon it. Slotted disc 16 is mounted to be rotated at a slow speed (3 revolutions per minute in the embodiment built), by motor 22. Motor 22 is mounted on the rear of plate 24.

Slots 26 are arranged near the periphery of disc 16 in a random manner both asto width of the slots and spacing between them. Mounted on the forward face of plate 24 are a plurality of photocells 28A-28C. In the fabricated embodiment, l6 Ratheon CKl24l cadmium selenide photocells were mounted in a circular arrangement with the same center and radius as 5 slots 26. Photocells 28A-28C have a resistance of 150 ohms real meters. The meter needle movement is the result of noise 20 in the system which is normally random both as to frequency of pulses as well as amplitude and disappears when the analog becomes zero. No economical method for simulating this noise was known.

SUMMARY OF THE INVENTION It is an object of this invention to provide a reliable and economical source of varying impedance.

It is a further object of this invention to provide such a source where the impedance will vary in a pseudorandom manner.

In a preferred form of the invention, a plurality of photoconductive devices (photocells) are positioned so that radiation from a light source will impinge upon them. A disc rotated at a slow speed, and having slots of random width and random spacing is interposed between the light source and the photocells so that the photocells will be. exposed to radiation for varying periods of time. Each photocell is connected to a circuit in parallel with a fixed resistance sized to achieve desired overall impedances when the photocells are illuminated and not illuminated. The varying impedance introduced into the circuit simulates noise.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram illustrating one application of the invention; and

FIG. 2 is a schematic isometric of one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a plurality of analog indicating devices l0A-10C, each having an internal resistance (in the actual indicators this was 5,000 ohms) represented by resistors l2A-I2C respectively is represented. Analog signals derived from sources 14A-l4C are provided to their respective indicators. The nature of the analog signals delivered by sources l4A-14C is such that, without additional circuitry, the indicator needle would move to the position designating the analog value without exhibiting noise in terms of the tremor or kick from the analog value normally found in many operating systems. In the case of the system actually built, this lack of noise was due to the fact that the analog signal wasartificially generated rather than originating in a transducer, sensor, or the like.

To better simulate the indicator movements found in an actual system, noise had to be added to the analog signals derived from sources 14A-14C with the following limitations:

1. When the analog signal was zero, no movement of the indicator pointer should occur.

2. The noise should appear to be random, and differ in pattern from indicator to indicator.

3. The magnitude of the noise should be such that it would cause changes of as little as 1 percent or as much as 3 percent in the impedance of the analog indicators.

when illuminated by foot candles and a resistance of 20,000 ohms or greater when not illuminated.

With the disclosed arrangement, therefore, as disc 16 rotates, photocells 28A-28C become illuminated for random periods of time at random intervals causing resistance changes between the illuminated and not illuminated values. To decrease the impingement of reflected radiation upon the photocells the reverse surface of disc 16 and the forward surface of plate 24 should be painted flat black. Ambient light is also minimized by making the section of enclosure 18 surrounding slotted disc 16 and plate 24 of an opaque material having a flat black interior surface coating. That part of enclosure 18 covering light source 20 is a perforated screen to protect against breakage .and allow ventilation. Maintenance is reduced by operating light source 20 below its rated voltage.

It should .be understood that light source 20 can be a source of visible, infrared, or ultraviolet light depending upon the photoconductive material employed in photocells 28A-28C.

Referring again to FIG. 1, photocells 28A-28C are connected in parallel to resistors BOA-30C. By selecting resistors 30A-30C having suitable resistances the desired percentage variation of the impedance can be obtained. For example, if about a 3 percent impedance variation were desired in the indicator reading of indicator 10C, resistor 30C would be chosen to be about 250 ohms. When photocell 28C is illuminated (having ohms resistance) the total impedance is:

Since the noise is not added to the analog signal produced, but instead varies the impedancethe signal experiences, when the signal goes to zero the indicator will not show noise. Also, by not wiring adjacent photocells to adjacent indicator circuits, but instead'using arandomarrangement the noise exhibitedbyadjacent indicators is out of phase and appears random.

While forthisparticular-use=a=random impedance variation was sought, it can be seen that other slot arrangements on disc .16, forexample a regularpatternof equally wide slots, could be used to introduce desired impedance variations into circuits.

Moreover, the basic invention can'be applied for other purposes. lf, for.example,.a pair of discs having slots initially aligned are mounted on a shaft,i-a relative displacement occurs when torque is applied causing a detectable impedance change. In a similar manner slotted discs mounted on independent coaxial shafts can be used to monitor synchronism or phase displacement.

I claim:

1. In a system having an analog indicating device for displaying the magnitude of a signal produced by a source, means for varying the impedance experienced by said signal in a pseudorandom manner comprising:

a fixed resistance connected between said source and said indicating device;

a source of radiation;

a photoconductive device positioned to have radiation from said source impinge upon it, and connected in parallel to said fixed resistance;

a disc positioned between said source of radiation and said photoconductive device having a plurality of slots of random width and random spacing arranged about its periphery for transmitting radiation to said device; and

means for rotating said disc so that radiation alternately impinges upon said photoconductive device and is blocked in a pseudorandom manner.

2. In a system having a plurality of analog indicating devices each displaying the magnitude of a signal produced by a source connected to it, means for varying the impedance experienced by each of said signals in a manner simulating noise comprising:

a fixed resistance connected between each of said indicating devices and its respective source;

a source of radiation;

a photoconductive device connected in parallel with each of said fixed resistances;

said photoconductive devices positioned in a circular pattern to have radiation from said source impinge upon them;

a disc positioned between said source of radiation and said photoconductive devices having a plurality of slots arranged around its periphery having the same radius and center as said photoconductive devices;

said slots being of random width and random spacing; and

means for rotating said disc so that radiation alternately impinges upon said photoconductive devices and is blocked in a pseudorandom manner. 

1. In a system having an analog indicating device for displaying the magnitude of a signal produced by a source, means for varying the impedance experienced by said signal in a pseudorandom manner comprising: a fixed resistance connected between said source and said indicating device; a source of radiation; a photoconductive device positioned to have radiation from said source impinge upon it, and connected in parallel to said fixed resistance; a disc positioned between said source of radiation and said photoconductive device having a plurality of slots of random width and random spacing arranged about its periphery for transmitting radiation to said device; and means for rotating said disc so that radiation alternately impinges upon said photoconductive device and is blocked in a pseudorandom manner.
 2. In a system having a plurality of analog indicating devices each displaying the magnitude of a signal produced by a source connected to it, means for varying the impedance experienced by each of said signals in a manner simulating noise comprising: a fixed resistance connected between each of said indicating devices and its respective source; a source of radiation; a photoconductive device connected in parallel with each of said fixed resistances; said photoconductive devices positioned in a circular pattern to have radiation from said source impinge upon them; a disc positioned between said source of radiation and said photoconductive devices having a plurality of slots arranged around its periphery having the same radius and center as said photoconductive devices; said slots being of random width and random spacing; and means for rotating said disc so that radiation alternately impinges upon said photoconductive devices and is blocked in a pseudorandom manner. 